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 Canards in military aircraft

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PostSubject: Canards in military aircraft   Tue Aug 28, 2012 7:59 am

Typhoon's canards

Typhoon is unstable long arm canard-delta design. Being statically unstable, it means that aircraft wants to pitch up all the time, and in level flight, canards are used to counteract that pitch up moment. However, when turning, canards are used as a control surface, pushing nose upwards, and actually contributing to lift.

Compared to delta-tail (e. g. F-16), Typhoon's canard configuration has several advantages. First advantage is that canard is effective at all angles of attack, whereas tail often has to cope with interference from wing at high AoA, unless it is far aft in relation to wing – required separation between wing an tail in turn reduces possible wing area, increasing wing loading.

Second advantage is connected to constant pitch-up force and center of gravity – when canard deflects upwards, it immediately cancels counter-force maintaining aircraft in stable flight, thus achieving good instantaneous turn rate. Moreover, due to statically unstable aircraft having center of gravity far aft, resulting in large lever arm effect, long arm canard does not have to impart as much force as tail to achieve the same effect.

Third is that during turn, tail will actually detract from lift, due to relatively large downforce required to achieve turn rate. Canard generates upforce, and requires less force for same effect, alleviating that problem.

Fourth advantage is that, with canards being located far forward at nose, wing can extend further aft, greatly increasing wing size, thus decreasing wing loading and improving sustained turn rate. Moreover, control surfaces located at wing trailling edge can be larger, although not entirety of control surfaces will be effective all the time.

There are some disadvantages, however. Center of gravity is located more aft, meaning that tailfin control surfaces are not very effective when steering aircraft left/right, requiring larger tailfin and thus higher drag.

Canard sizing is also extremely sensitive compared to tail – it is easy to get canard that will harm maneuverability.


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PostSubject: Close-coupled canards and LEX/LERX   Wed Aug 29, 2012 6:27 pm

Unlike long-arm canards, close coupled canards' primary function is not that of a control surface. Instead, they perform function similar to LERX (Leading Edge Root Extension), creating vortices which help connect airflow to surface of the wing at high angles of attack (it is important to note that, with delta wing, such vortices always exist; canard/LEX are used simply to strenghten them).

Result is that effective angle of attack at wing is reduced, allowing aircraft to achieve higher angle of attack without wing stalling, and thus turn tighter tighter.

When designed properly, interaction between vortices from close coupled canard and LEX can strenghten vortices themselves, increasing lift gain beyond what is achievable by either canards or LEX of same size as those used.
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